Recently, development work has been done on a membrane supported bioreactor concept. For example, U.S. Pat. Nos. 4,181,604 and 4,746,435 describe a process for treating wastewater by supplying oxygen from one side of a gas-permeable membrane to micro-organisms growing on the other side of the membrane. Hollow fibers with porous walls were used as the membrane. In U.S. Pat. No. 5,116,506, a gas permeable membrane divides a reactor vessel into a liquid compartment and a gas compartment. A biofilm is grown on the gas permeable membrane on the liquid side of the membrane. Oxygen and alternate gases pass through the membrane to the bacteria growing on the liquid side of the membrane.